This invention relates to a method of inspecting liquid crystal display cells in TFT arrays after filling a liquid crystal therein.
Generally, TFT liquid crystal displays must be inspected for irregularities in arrays, empty cells and display cells filled with a liquid crystal that are assembled into liquid crystal modules of individual pixels.
The inventor proposed a technology related to a method of inspecting the arrays and cells of TFT liquid crystal displays before they are assembled into modules in Japanese Patent Application No. 133796 of 1993.
This technology essentially relates to a method of inspecting each cell of an active color TFT liquid crystal display (LCD) of the type shown in FIG. 1. The active color TFT LCD is connected to a grounded auxiliary pixel capacitor C.sub.s to which the drain side of a TFT is connected in an equivalent circuit of the type shown in FIG. 2. With the TFT energized for a certain length of time, the cell capacitor of the LCD is charged through the data line, as shown in FIG. 3. After de-energizing the TFT for a certain length of time and maintaining the charged condition, the TFT is re-energized to release the electric charge through the source and drain of the TFT and the resistor connected to the ground side thereof. Then, whether wiring is normally connected or not is judged from a drop in the voltage output from a discharge current detecting resistor.
Inspections of the TFT arrays and the condition of their connections are completed before display cells are formed by filling in a liquid crystal in individual pixel cells. Thus, whether or not the cells filled with liquid crystal are normal or a liquid crystal has been properly filled in individual cells is determined by checking if the pixel capacitor C.sub.LC provided between the pixel and opposed electrodes of the TFT in each cell filled with a liquid crystal shows a normal discharge.
Conventionally, whether or not the pixel capacitor C.sub.LC shows a normal discharge has been checked by examining the condition of the pixel capacitor C.sub.LC alone by cutting off the connection with the ground of the auxiliary pixel capacitor C.sub.s, as shown in FIG. 4(a).
However, this method does not permit inspecting the pixel capacitor C.sub.LC alone for the reason given below.
Actually, very large stray capacitances exist between the auxiliary pixel capacitor C.sub.s and the opposed electrode 8 and adjoining gate line G.sub.i+1, as shown in FIG. 4(b). Therefore, the condition of the pixel capacitor C.sub.LC remains the same as one with the auxiliary pixel capacitor C.sub.s even when connection between one end thereof and the ground is cut off.
In FIG. 4(b), C.sub.F1 designates a stray capacitance between the terminal S.sub.T of a connecting line connected to the auxiliary pixel capacitor C.sub.s and floating away from the ground and the opposed electrode 8, whereas C.sub.F2 denotes a stray capacitance between the terminal S.sub.T and the adjoining gate line G.sub.i+1. The auxiliary pixel capacitor C.sub.s is connected to the ground through the stray capacitances C.sub.F1 and C.sub.F2. The stray capacitances C.sub.F1 and C.sub.F2 are more than 100 times larger than the capacity of the auxiliary pixel capacitor C.sub.s. Even if the auxiliary pixel capacitor C.sub.s is cut off from the ground with a view to inspecting the pixel capacitor C.sub.LC alone, the influence of the auxiliary pixel capacitor C.sub.s remains uneliminated.
The total capacitance between the drain and ground of a TFT can be expressed as C.sub.LC +C.sub.s (C.sub.F1 +C.sub.F2)/(C.sub.s +C.sub.F1 +C.sub.F2). Because C.sub.F1, C.sub.F2 &gt;C.sub.s, the total capacitance becomes substantially equal to C.sub.LC +C.sub.s. This means that cutting off of the auxiliary pixel capacitor C.sub.s from the ground produces little effect. (In FIG. 4(b), stray capacitance exists between the adjoining gate line and opposed electrode. However, the influence of the stray capacitance therebetween is negligible because the adjoining gate line and opposed electrode are connected to the ground and, therefore, there is no voltage difference therebetween.)
The object of this invention is to provide a method of realizing a charging and discharging inspection of the pixel capacitor C.sub.LC alone by eliminating the shortcomings of conventional inspection methods of the type shown in FIG. 4(a).